There is a need in liquid chromatography for solvent pump systems which can deliver fluids in accurately metered amounts. It is important in such systems that solvent flow be precisely controlled independently of the load pressure. Liquid chromatography systems and particularly systems utilizing gradient elution require high accuracy at extremely low flow rates. For example, in micro-bore chromatography it is desirable to resolve flow rates as low as one microliter per minute. It has been a problem with most prior art devices that the compressibility of the fluid and the mechanical compliance of the pump combine to cause a severe drop in flow rate as load pressure increases, this phenomenon being commonly referred to as "roll off". A two pump system overcomes inaccuracies due to roll off by utilizing a low pressure meter pump which injects controlled amounts of solvent into a high pressure slave pump. The meter pump is synchronized with the slave pump to insure that the meter pump delivers its metered charge into a low pressure portion of the slave pump cycle. Because the meter pump always operates into a low pressure load, roll off is not a problem affecting meter pump accuracy. Such a pump system is disclosed in U.S. Pat. No. 4,003,679 HIGH PRESSURE PUMP WITH METERING by Douglas McManigill and assigned to the assignee of the present application.
The McManigill specification describes a system in which the meter pump/slave pump pair are mechanically coupled to cycle at the same fixed rate. Flow rate is controlled by mechanically adjusting meter pump displacement. However, mechanical and fluid compliance in the meter pump introduces some error in each meter pump cycle. This error tends to increase as a function of increasing meter pump cycle rate. As flow rate and hence meter pump displacement is reduced this error and others inherent in scaling become an increasingly greater percentage of the volume of fluid pumped. Any such error is reproduced in each cycle of the meter pump/slave pump pair and reduces system flow rate accuracy, particularly at low flow rates. Hence, the range of flow rates over which a given meter pump can operate is limited by degradation of accuracy. Difficulty in self-priming may also become a problem at low flow rates in pumps that adjust flow rate by controlling displacement because larger displacement pumps tend to self-prime better than small displacement pumps.